Hate potholes? Nuke 'em

In a sense, we're still living in the olden days of pothole repair, when most are fixed by the "throw and go" process.

Its name says it all; just throw in the hot mix of fill material and move on to the next hole.

And you'll probably have to do the same thing with the same potholes next year—if not next week.

But with a little help from microwaves and magnetite, the olden days may soon give way to a golden age of long-lasting pothole repairs. Research led by Lawrence Zanko, a senior research fellow at the University of Minnesota Duluth's Natural Resources Research Institute (NRRI), indicates that mixing ground magnetite—an iron mineral found in taconite ore—into the asphalt patch material and then "nuking" it with microwaves will seal the patch far more securely than conventional methods.

"The magnetite we use in the repair mix enhances the absorption of microwave energy, which leads to faster heating of the repair," says Zanko. "The magnetite also helps prevent the transmission of microwave energy more deeply into the ground, keeping more energy within the repair."

Ten years in the making

Zanko got the idea for using microwaves and magnetite about 10 years ago from a colleague named David Hopstock, who had worked in the U.S. Bureau of Mines in the Twin Cities.

"He said the magnetite within taconite materials was an excellent microwave absorber, and he had ideas for applying microwaves for deicing pavements without chemicals and for pothole and pavement repair," Zanko recalls. The research project gradually grew, including experiments with a lawnmover-sized device and a scaled-up test chamber in Duluth that is, literally, a "walk-in" microwave oven.

The two eventually contaced Microwave Utilities Inc. (MUI), a Monticello-based company that had a prototype mobile technology to use microwaves to thaw frozen ground. Working with company designers and engineers, the Duluth researchers pursued a project with Anoka and St. Louis counties to test the microwave + magnetite technology for pothole repair.

Results were impressive.

"We did some repairs in an Anoka main street in April 2011, and they were holding up a year later," says Zanko. Current side-by-side studies with conventional repairs indicate improved durability and longevity, but it's too early to calculate the full lifetime of an "M + M" repair.

On-the-spot fix

The technology starts with removing excess water and debris from a pothole and adding a mix of ground-up recycled asphalt pavement and shingles and powdered magnetite (1-2 percent by weight). A 50-kilowatt generator on an MUI truck produces microwaves; they travel through an aluminum tube (called a waveguide) to a stainless steel box that is placed over the pothole and surrounding pavement. Flanges seal the box against microwave leakage.

Within minutes, microwave heating raises the repair area temperature to 250-300 F. With mixing and compacting, the fill binds well to the existing pavement.

"What I like about it is that it's particularly effective in winter, when so many other repair options don't work too well," Zanko says. "A week later those other repairs may be failing or gone."

Also, metal utilities like valve shutoffs and manhole structures reflect the microwaves, so they don't heat up if an adjacent pothole is undergoing repair.

Zanko and his colleagues are also working on a new taconite-containing repair mix formula that sets in 10 to 15 minutes—without any microwaves. The Minnesota Department of Transportation is watching closely as they evaluate the two technologies. Zanko is optimistic that at least one will become standard for all seasons.

"We consider this to be semi-permanent to permanent," he says. "Currently, if a repair lasts the season, it's considered a success. But we think that when these [microwave + magnetite] repairs are done, they're done."